Measuring

measure something. draw box in design program sized to that something. scale that box to the known size of that box.

edit: i dont know how to scale in illy, but in corel you double click the ruler then hit scale, then type in size of box (say 2.542 inches) = known size of box (say 35.5 inches) then the document is now scaled correctly

 

+1
Also take actual measurements of what your wrapping to make sure everything is as it should be in your scaled version.

 

vehicle templates.. then double check with measuring tape?

 

Sticky Yard is decent. Although I will take a picture and measure something I can scale in Illustrator. For vehicles I measure a rim.